make sure snapshot is valid

Author: Jan Xie

ethereum/snapshot.py

@@ -44,6 +44,8 @@ def get_ancestor_list(self, n):
 
 def create_snapshot(chain, recent=1024):
     env = chain.env
+    assert recent > env.config['MAX_UNCLE_DEPTH']+2
+
     head_block = chain.head
     base_block_hash = chain.index.get_block_by_number(max(head_block.number-recent, 0))
     base_block = chain.get(base_block_hash)
@@ -197,4 +199,4 @@ def snapshot_form(val):
     if is_numeric(val):
         return str(val)
     elif is_string(val):
-        return b'0x' + encode_hex(val)
+        return b'0x' + encode_hex(val)